1. Field of the Invention
This invention relates to an apparatus for providing a substantially orthogonal transition between a coaxial line section and a stripline conductor or conductors suspended in air between two ground planes, where the center conductor of the coax is connected to the stripline conductor or conductors, and the other conductor of the coax is connected to each of the two ground planes. The device has particular applicability to microwave antenna feed networks such as are found in communications satellites. Air stripline transmission lines are widely used in microwave circuits because they are much less lossy than other types of transmission lines such as coaxial cable (the loss increases as the R.F. frequency increases). Optionally, two conductors sandwiching a dielectric are employed, rather than a single conductor, to cut the power loss even further and to provide thermal balance, which is important when the network will be utilized in an environment with widely varying temperatures, such as a communications satellite.
2. Description of the Prior Art
Air stripline conductor antenna feed networks are described in Gish and Graham, "Characteristic Impedance and Phase Velocity of a Dielectric-Supported Air Strip Transmission Line with Side Walls," IEEE Transactions on Microwave Theory and Techniques, Volume MTT-18, No. 3, March 1970, p. 131 et seq., and in Davis, "Air Stripline Promises to Cut Cost of Phased Arrays," Microwaves, October 1973, p. 12 et seq.
U.S. Pat. No. 3,681,769 and U.S. Pat. No. 3,854,140 shows connections between coaxial cables and dual strip conductors wherein the center conductor of the coax is connected to one of the strips and the other conductor of the coax is connected to another of the strips.
U.S. Pat. No. 3,337,820 shows an orthogonal connection between a coaxial connector and a strip conductor which is not suspended in air as in the present invention but is sandwiched between solid dielectrics.
Keen, "Scientific Report on Study of Strip Transmission Lines," Cambridge Air Force Research Center Contract AF19(604)-780, Report No. 2830-2, Dec. 1, 1955, Airbone Instruments Laboratory, Inc., Mineola, New York, in FIG. 22 shows an orthogonal connection between a coaxial connector and a single strip conductor with no solid dielectric between the ground planes. This reference does not show the dam structures of the present invention.
U.S. Pat. Nos. 2,964,718 and 2,968,012 are of background interest.
None of the above references discloses a substantially orthogonal connection between a coaxial connector and a stripline conductor (or identical dual stripline conductors sandwiched around a dielectric) suspended in air between two ground planes, wherein the center conductor of the coax is connected to the stripline conductor(s) and the outer conductor of the coax is connected to each of the two ground planes, as in the present invention. Early attempts by the inventors to make such a transition (and a transition in the case where a single strip conductor is employed) revealed the following serious problems:
1. The side-launch transition geometry presented a discontinuity in the non-homogeneous air stripline transmission line, launching parallel plate modes causing mismatches, phase deviations and R.F. losses at various points within the frequency band of interest.
2. The side-launch transition geometries presented high voltage standing wave ratios to the output ports, particularly at microwave frequencies such as C-band.
3. In the case of dual stripline conductors, the side-launch transition generated spurious modes propagating between the strip conductors within the dielectric support, also manifesting themselves in the form of mismatches, phase deviations and R.F. losses at various points within the frequency band of interest.
These problems are particularly acute when the application for the side-launch transition is in a network feeding a frequency-reuse shaped-beam antenna system, such as would be employed in a communications satellite.